LEDs with an overall high luminance are useful in backlighting for Liquid Crystal Display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display. In a large LCD matrix display, typically, the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current. Matrix displays typically display the image as a series of frames, with the information for the display being drawn from left to right in a series of descending lines during the frame.
In order to supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized as a luminaire, the white LEDs typically comprising a blue LED with a phosphor, which absorbs the blue light emitted by the LED to emit a white light. In a second technique one or more individual strings of colored LEDs, functioning as a luminaire, are placed in proximity so that in combination their light is seen as white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs. Each of the colored LED strings is typically intensity-controlled by Pulse Width Modulation (PWM) to achieve an overall fixed perceived luminance and white point balance. The current, when pulsed on, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current.
Overall luminance is controlled by changing the PWM duty cycle of each color multiplied by a common factor while the white balance point is maintained by the proportion between the three color PWM duty cycle signals. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the initial white point.
The colored LEDs also change their output as a function of temperature. The LED changes are corrected by adjusting the respective PWM duty cycles with a color loop controller. It is to be noted that changes to the color LED output are relatively slow, particularly as compared to frame time.
A known problem of LCD matrix displays is reduced contrast caused by light leakage through the orthogonal polarizers of the LCD display, particularly in the presence of ambient light. This problem is addressed by adding dynamic capability to the backlight. The dynamic capability adjusts the overall luminance of the backlight for each zone responsive to the current video signal, typically calculated by a video processor. Thus, in the event of a dark scene, the backlight luminance is reduced thereby improving the contrast. Since the luminance of a scene may change on a frame by frame basis, the luminance is preferably set on a frame by frame basis, responsive to the video processor. It is to be noted that a new frame begins every 16.7-20 milliseconds, depending on the system used.
An article by Perduijn et al, entitled “Light Output Feedback Solution for RGB LED Backlight Applications, published as part of the SID 03 Digest, by the Society for Information Display, San Jose, Calif., ISSN/0003-0996X/3/3403-1254, the entire contents of which is incorporated herein by reference, is addressed to a backlighting system utilizing RGB LED light sources, a color sensor and feedback controller operative to maintain color stability over temperature fluctuations. Optionally, brightness can be maintained constant. Brightness, or luminance, control is accomplished by comparing the luminance sensed output of the LEDs with a luminance set point. The difference is fed to a PI compensator duty control whose output is multiplied with the input set points, and the loop is closed via the color control loop. Unfortunately, in the instance of a dynamic backlight as described above, use of the color control loop to control luminance requires a high speed color loop, because the luminance may change from frame to frame. Such a high speed color loop adds to the cost.
U.S. Patent Application Publication S/N 2006/0221047 A1 in the name of Tanizoe et al, published Oct. 5, 2006 and entitled “Liquid Crystal Display Device”, the entire contents of which is incorporated herein by reference, is addressed to a liquid crystal display device capable of shortening the time required for stabilizing the brightness and chromaticity in response to a temperature change. A brightness setting means is multiplied with a color setting means prior to feedback to a comparison means, and thus a single feedback loop controls both brightness and color. Unfortunately, in the instance of dynamic backlight, use of the color control loop to control luminance requires a high-speed color loop, because the luminance may change from frame to frame, thus adding to the cost.
What is needed, and not provided by the prior art, is a color controller for a luminaire whose target luminance and/or color may vary on a frame to frame basis, without requiring a high speed color control loop.